In conventional circuit-switched telephony networks, such as the Public Switched Telephony System (“PSTN”), each user endpoint is connected to at most one intelligent switching point. For example, in a typical business enterprise, a business telephone is connected to exactly one Private Branch Exchange (“PBX”). A PBX is an intelligent switching point within a circuit-switched network that is responsible for routing calls to and from user endpoints or telephones that are physically connected by copper wires to the PBX. As a practical matter, since a physical connection is required between a user endpoint and an intelligent switching point, the user endpoint is only connected to at most one intelligent switching point. This requirement is a limitation of circuit-switching technology.
Newer telephony networks that employ packet-switching technologies are growing in popularity. In particular, packet-switched telephony networks that use the Internet Protocol (“IP”) as a network protocol are becoming prevalent. These so-called Internet telephony networks have a potential to offer new features and services that are currently unavailable to users of circuit-switched telephony networks. Networks that employ both conventional telephony and Internet telephony are said to be converged networks.
The Session Initiation Protocol (“SIP”) is one of several protocols that may be used in conjunction with the Internet Protocol to support Internet Telephony applications. The SIP specification is defined in the Internet Engineering Task Force (“IETF”) Request for Comments (“RFC”) 3261, dated June 2002; the disclosure of which is incorporated herein by reference in its entirety. SIP is an application-layer control protocol for creating, modifying, and terminating sessions between SIP endpoints, which are referred to as SIP user agents. A user of a SIP-enabled device can maintain a single externally visible logical identifier that is referred to as a SIP address-of-record, which allows the user to be contacted regardless of her physical network location. SIP enables a user agent to discover one or more network locations or user agents that are associated with a particular address-of-record. SIP also allows user agents to negotiate parameters that are associated with media sessions.
Typically, each SIP application user in a SIP-based network is assigned a SIP address-of-record. A SIP address-of-record is specified using a SIP Universal Resource Identifier (“URI”). The format of a SIP URI is similar to that of an email address, which typically includes a user name “at” a network domain name, for example “sip:alice@siemens.com.” The SIP specification also defines a SIPS URI, for example “sips:alice@siemens.com.” When a SIPS URI is used, a SIP user agent that is associated with the SIPS URI should be contacted securely, for example using the Transport Layer Security (“TLS”) protocol.
The SIP specification defines several types of communication resources, which include SIP Registrars, SIP Redirect Servers, and SIP Proxy Servers. These SIP communication resources are responsible for sending, receiving, routing, and relaying SIP messages among SIP user agents, so that SIP-based sessions may be established, modified, and terminated.
SIP Proxy Servers perform a variety of functions in SIP-based networks, such as helping to route a SIP-based session invitation message to a particular SIP network user's current network location(s), authenticating and authorizing SIP user agents for particular services, implementing message-routing policies, and providing features to SIP network users.
SIP registration is a mechanism that creates a SIP address binding in a Location Service for a particular network domain. A SIP address binding associates a SIP address-of-record with one or more contact addresses. A SIP user agent can register a SIP address-of-record with a SIP Registrar to associate the SIP address-of-record with the SIP user agent's network address. SIP Registrars process SIP REGISTER request messages that are received from SIP user agents to add, remove, and query address bindings.
For example, a SIP address-of-record of “sips:alice@siemens.com” may be bound to a contact address of “sips:alice@dev101.research.siemens.com.” A SIP address binding maps an incoming SIP address-of-record to one or more contact addresses. When a valid SIP REGISTER request message, which adds a SIP address binding, is received by a SIP Registrar, the SIP Registrar instructs a Location Service for a network domain to store the address binding. The Location Service allows other SIP user agents in the network domain to discover the contact address associated with the SIP address-of-record. In this regard, a SIP Registrar acts as an intelligent switching point within a packet-switched network that is responsible for routing media sessions among SIP user agents.
Prior art SIP-enabled devices have not had an ability to register with more than one SIP Registrar. As is the case with conventional telephones in a circuit-switched network, prior art SIP-enabled devices are only associated with a single intelligent switching point. In this regard, prior art SIP-enabled devices suffer from many of the same limitations of conventional telephones in circuit-switched networks.